Heretofore, the photocatalytic reduction of carbon dioxide into various organic compounds such as formaldehyde and methanol has been accomplished using various chemical compounds such as titanium dioxide, tungsten trioxide, lead oxide, iron oxide, calcium titanate, silicon carbide, and the like. See "Photoreduction of Carbon Dioxide and Water into Formaldehyde and Methanol on Semiconductor Materials" by Aurian-Blajeni, Halmann and Manassen, Solar Energy, Vol. 25, pp. 165-170, 1980. This photocatalytic process does not employ any physically separate electrodes or special electrolyte as does the classical electrolytic cell or photoelectrochemical cells as will be discussed in greater detail hereinafter. This photocatalytic process merely employs a catalytic material, preferably, dispersed in a carrier liquid for better carbon dioxide contacting purposes. The material to be reduced such as carbon dioxide, is brought into contact with the catalyst using light as a source of the energy of reduction.
Heretofore, such photocatalytic processes have relied on chemical compounds rather than individual elements or metals based on thermodynamic considerations. For example, it was thought that silicon would not work in a photocatalytic context because, based on photoconductor theory, silicon (elemental silicon), when excited by photons would not have sufficient energy to reduce carbon dioxide in a water carrier.
Also heretofore, photoelectrochemical cells which employ two physically separate electrodes combined with a special electrolyte solution have been used to reduce carbon dioxide or the bicarbonate ion to organic compounds such as formaldehyde, methanol, and formic acid. These cells have employed silicon metal as one of the physically separate electrodes and carbon or the like as the counter electrode. In the operation of these cells at least part of the required energy of reduction is supplied by light energy, including solar radiation. See U.S. Pat. No. 4,219,392, issued Aug. 26, 1980 to Halmann.
Also heretofore, the photoreduction of nitrogen to ammonia has been reported using titanium dioxide alone or in combination with oxides of Fe.sub.2 O.sub.3, cobalt, molybdenum, and nickel. See "Photolysis of Water and Photoreduction of Nitrogen on Titanium Dioxide", Journal of the American Chemical Society, Volume 99, pp. 7189-7193, 1977. This reference also discloses that enhancement of ammonia production was not obtained where numerous other metals such as chromium, copper, palladium, silver, and vanadium were used, thereby demonstrating the lack of predictability in this area.